1. Field of the Invention
The present invention relates to a nonlinear optical device and an optical signal processing unit for use in the fields of optoelectronics, optical information processing, optical communication, etc.
2. Description of the Prior Art
Nonlinear optical materials are materials having a nonlinear response of second or higher order in a strong electric field of a laser beam and are important to optical signal processing such as frequency conversion, oscillation and switching.
In particular, third-order nonlinear optical materials have attracted interest as a basic material in future optical communication and information processing wherein excellent characteristics inherent in light, that is, high speed and parallel processing are sufficiently exhibited.
Among the nonlinear optical materials, organic nonlinear optical materials are particularly important because some of them exhibit higher response speed and larger nonlinear optical constant than those of the conventional inorganic nonlinear optical materials.
Although the mechanism through which the third-order nonlinear optical effect is developed has not been elucidated yet, it is known, for example, that organic nonlinear optical materials having an extended delocalized .pi. electron system exhibit excellent third-order nonlinear characteristics.
Aromatic compounds comprising linearly linked aromatic rings are known as the organic nonlinear optical material having a delocalized .pi. electron system. These aromatic compounds, however, had a problem that an increase in the number of aromatic rings deteriorates the heat stability and causes the absorption wavelength of light to shift to a longer wavelength side.
A nonlinear optical device wherein use is made of a third-order nonlinear optical material utilizes a change in the refractive index caused when exposed to light.
Amplification of a minute change in the refractive index through the use of, for example, a Fabry-Perrot resonator is known as a method of reading the change in the refractive index. In this method, however, even a slight instability in the light source sensitively affects the resonance stability, which renders the whole system very delicate. Therefore, a high accuracy of dimension and quality necessary for stably operating the system poses an obstacle to the cost and mass productivity. Further, a very high energy should inevitably be applied for the purpose of increasing the change in the refractive index, which brings about problems of heat resistance of material, heat barrier, thermal effect, technical barrier associated with loading of information on the high applied energy, etc.
A proposed method for solving the above-described problem is to conduct detection with a very high sensitivity through the measurement of elliptically polarized light by means of weak probe light.
This method comprises inducing optical anisotropy in a substance with an intense pumping light to cause a change in the polarized light in a linearly polarized signal light.
In this method, in order to utilize the optically induced optical anisotropy, it is necessary to use means such as the use of a circularly polarized light as the pumping light or the inclination of the direction of polarization of the pumping light from the direction of polarization of the signal light, which limits the signal processing method.